The present invention relates in general to apparatus for producing plastic pipes, and more particularly to an apparatus for producing plastic pipes by the extrusion process in which the thickness of the plastic pipe is controlled by the takeaway speed of soft plastic pipe.
In the patent to Graves, et al., U.S. Pat. No. 4,137,025, issued on Jan. 30, 1979, for Apparatus For Measuring And Controlling The Wall Thickness Of Plastic Pipes and the patent to Graves, et al., U.S. Pat. No. 4,152,380 issued on May 1, 1979, for Method Of And Apparatus For Measuring And Controlling The Wall Thickness Of Plastic Pipes, there is disclosed an extrusion process for the manufacture of plastic pipes in which the inside diameter of the pipe and, therefore, the wall thickness of the pipe is controlled by the takeaway speed at which the molten plastic pipe is removed from the sizing sleeve. An ultrasonic transducer is disposed at the upstream end of the sizing sleeve to provide signals representative of the thickness of the molten pipe advancing in the sizing sleeve. These signals are sent to devices for comparing the measured thickness of molten plastic pipe with the desired thickness of the molten plastic pipe for producing a correction signal. The correction signal is employed for correcting the takeaway speed at which the molten plastic pipe is removed from the sizing sleeve.
In the patent to Boggs, et al., U.S. Pat. No. 3,916,676, issued on Nov. 4, 1975, for Method Of And Apparatus For Measuring Automatically Successive Sections of An Elongated Material, there is disclosed four crystal transducers spaced equal angular distances about the periphery of an advancing cable. The cable is immersed in water and the crystal transducers extend into the water. The crystal transducers are pulsed sequentially for measuring the thickness of the jacket of the cable in successive sections. In the patent to Boggs, et al., U.S. Pat. No. 3,827,287, issued on Aug. 6, 1974, for Method Of And Apparatus For Measuring The Thickness Of Successive Sections Of A Cable Jacket, there is disclosed a pulsed crystal transducer for measuring ultrasonically successive sections of a cable having a jacket for measuring the thickness of the jacket.
The patent to Saint-Amour, U.S. Pat. No. 4,520,672, issued June 4, 1985, for Thickness Measuring discloses a system for measuring the wall thickness of flexible tubing through ultrasonic techniques. Four ultrasonic probes are spaced about the tube in quadrature relation. The probes are disposed in water surrounding the flexible tube.
The patent to Livingstone, U.S. Pat. No. 4,487,072, issued on Dec. 11, 1984 for Ultrasonic Testing Of Tubular Goods discloses a test head with circumferential array of transducers oriented for flow inspection of non-rotating goods. The patent to Falgari, et al., U.S. Pat. No. 4,089,227, issued on May 16, 1978, for Apparatus For Measuring The Radial Dimensions Of A Cylindrical Tube By Ultrasonics discloses an ultrasonic transducer whose emitting part is shaped like a ring for emitting pulses to measure the radial dimensions of a tube by ultrasonics.
Krautkramer Branson Company of Lewistown, Pa., manufactured and sold an Ultrasonic Thickness Measurement System For Production Monitoring And Control (WDM-U) in which a probe transmitted a sound pulse at regular intervals through a delay medium into the piece to be tested. An interface echo and a backwall echo were returned to the probe. The transit time of the sound pulses was measured between the two pulses as the sound pulse travels through the test piece.
Panametrics Ultrasonic Thickness Gauging System (5215 Series) manufactured and sold by Panametrics, Inc. of Waltham, Massachusetts used an ultrasonic pulse-echo technique. Short duration electrical pulses were sent to a piezoelectric transducer and the transducer converted the pulses into short bursts of high frequency sound energy. The sound energy was transmitted into the test material through water and through the test material and was reflected from the inside surface of the test material. The reflected sound waves were returned to the transducer and were converted into electrical pulses.
LFE Corporation of Waltham, Massachusetts manufactured and sold the LFE System 535 for measuring and controlling the wall thickness of extruded plastic pipe lines. The system used ultrasonic sensors. For small diameter vacuum-sized pipes, four fixed probes were mounted at the sizing sleeve and were either submerged in water or operated in a spray tank. For larger diameter pressure-sized pipes, a moving probe scanned the entire circumference of a pipe by reversing direction of travel along the circumference of the pipe. The scanning probe was mounted at the sizing sleeve in a cooling tank and was either used in a spray tank or was submerged in water.
In the manufacture of plastic pipes by the extrusion process, the wall thickness of the pipe is controlled by the takeaway speed at which the plastic pipe is removed from the sizing sleeve. Sizing sleeves have been made of metal. A metal sizing sleeve presents an acoustical impedance mismatch between the plastic pipe wall and the metal sizing sleeve during the transmission of ultrasonic sound. A large impedance mismatch between the metal sizing sleeve and the plastic pipe results in substantial energy dissipation of the ultrasonic sound at the interface between the metal sizing sleeve and the plastic pipe.
In the manufacture of plastic pipes by the extrusion process, the ultrasonic sound has been generated by a transducer and transmitted to the plastic pipe through a sizing sleeve and water. The transducer and the sizing sleeve may be submerged in cooling water under vacuum in a vacuum sizing/cooling tank. Heretofore a separate vacuum cooling water system was installed for the sizing sleeve.